1. Field of the Invention
The present invention generally relates to a method of transferring data and an earth station used in satellite communication.
Satellite communication systems are widely used by corporations, government agencies, etc. Use of such systems is not limited to use as a backup system, but includes delivery of information from a base station to a large number of stations when the information is of such a broadcast nature. In particular, such systems are used for simultaneous delivery of identical data to a large number of different locations.
2. Description of the Related Art
FIG. 13 is an illustrative drawing showing a configuration of a satellite communication system in which data is first delivered via a satellite network, and is resent via a surface communication line. Namely, both satellite communication and surface communication are used.
The communication system of FIG. 13 includes a surface-station center 30-0, an earth transmission station 21-0, and earth receiver stations 21-1-21-n. The surface-station center 30-0 generates data to be delivered. The earth transmission station 21-0 transmits the delivery data via the satellite communication link. The earth receiver stations 21-1-21-n receive the delivery data via satellite communication links.
Operations of the communication system will be described below.
The surface-station center 30-0 sends delivery data to the earth transmission station 21-0 via a surface communication line. The earth transmission station 21-0 delivers the data simultaneously to the earth receiver stations 21-1-21-n through satellite communication links (i.e., through an uplink, a satellite, and downlinks). Each of the earth receiver stations 21-1-21-n receives the delivered data.
If one of the earth receiver stations receives the delivery data with some errors, this receiver station sends a request to the surface-station center 30-0 via a surface communication line to have the delivery data transmitted again. In response, the surface-station center 30-0 resend the delivery data to this earth receiver station via the surface communication line. Depending on circumstances surrounding the retransmission requests, the surface-station center 30-0 may use the earth transmission station 21-0 to resend the data via the satellite communication links. This scheme is disclosed in Japanese Patent Laid-open Application No. 5-167565.
FIG. 14 is an illustrative drawing showing another configuration of a satellite communication system in which data is first delivered via a satellite network, and is resent via a surface communication line.
In this satellite communication system, an auxiliary earth transmission station is provided in order to attend to some of what the surface-station center 30-0 does in the configuration of FIG. 13. One of the earth receiver stations 21-1-21-n (i.e., the earth receiver station 21-1 in the example of FIG. 14) is assigned as the auxiliary earth transmission station. The earth receiver station 21-1 accumulates all the received delivery data, and sends a relevant portion of the data to another earth receiver station via a surface communication line when this receiver station requests retransmission of delivery data.
In this manner, if one of the earth receiver stations receives the delivery data with some errors, this receiver station sends a request to the auxiliary earth transmission station 21-1 via the surface communication line to have the delivery data transmitted again. This scheme is disclosed in Japanese Patent Laid-open Application No. 7-202779.
FIG. 15 is an illustrative drawing showing a configuration of a satellite communication system in which data delivered via a satellite network is resent via surface communication lines from a plurality of auxiliary earth transmission stations.
In the satellite communication system of FIG. 15, auxiliary earth transmission stations 30-1-30-n serving the same purposes as the auxiliary earth transmission station 21-1 of FIG. 14 are provided. Each of the auxiliary earth transmission stations 30-1-30-n is connected and dedicated to a separate set of the earth receiver stations 21-1-21-n. Further, the auxiliary earth transmission stations 30-1-30-n are connected to the earth transmission station 21-0 via a surface communication line.
Each of the auxiliary earth transmission stations 30-1-30-n accumulates delivered data that is received from earth transmission station 21-0 not only through the communication satellite but also through the surface communication line. In response to a retransmission request from an earth receiver station, a corresponding auxiliary earth transmission station resends the delivery data to the requesting station via a surface communication line.
In this manner, if one of the earth receiver stations receives the delivery data with some errors, this receiver station sends a request to a corresponding auxiliary earth transmission station via the surface communication line to have the delivery data transmitted again. This scheme is disclosed in Japanese Patent Laid-open Application No. 7-202781.
In the data transfer of the related-art satellite communication systems, an uplink between the earth transmission station 21-0 and the communication satellite may suffer a degradation in communication link quality (i.e., a decrease in carrier-to-noise power ratio) owing to signal attenuation caused by rains or the like. Such a degradation may result in bit errors occurring in the delivery data, and may cause earth receiver stations to receive erroneous delivery data. It is in such a situation when retransmission of data needs to be requested.
If a large number of earth receiver stations fail to receive correct data, all of these receiver stations issue a request for retransmission of delivery data to the transmission station (i.e., the surface-station center 30-0, the auxiliary earth transmission station 21-1, or the auxiliary earth transmission stations 30-1-30-n). As a result, the load of retransmission requests is concentrated on the transmission station. If a surface communication line connected to the transmission station is out of operation because of a natural disaster, a transit-device failure etc., the delivery data can not be retransmitted and corrected in the large number of earth receiver stations.
Accordingly, there is a need for a method of data transfer which can prevent data-transfer requests from concentrating on a single station when a large number of earth receiver stations try to have the failed-delivery data resend for correction purposes.
Accordingly, it is a general object of the present invention to provide a method of data transfer which can satisfy the need described above.
It is another and more specific object of the present invention to provide a method of data transfer which can prevent data-transfer requests from concentrating on a single station when a large number of earth receiver stations try to have the failed-delivery data resend for correction purposes.
In order to achieve the needs described above according to the present invention, a method of delivering data from a transmission station to receiver stations via satellite communication links includes the steps of delivering delivery data from the transmission station to the receiver stations via the satellite communication links, checking if the delivery data is successfully received at each of the receiver stations, sending results of the check from the receiver stations to the transmission station, generating control data from the results of the check at the transmission station, the control data indicating which receiver station failed to receive the delivery data and which receiver station succeeded in receiving the delivery data, sending the control data from the transmission station to the receiver stations, and transferring the delivery data in response to the control data from a first one of the receiver stations having succeeded in receiving the delivery data to a second one of the receiver stations having failed to receive the delivery data.
In the method described above, the second one of the receiver stations may send a transfer request to the first one of the receiver stations in response to the control data, thereby having the first one of the receiver stations transfer the delivery data to the second one of the receiver stations. Alternatively, the first one of the receiver stations may initiate the transfer of delivery data in response to the control data without receiving a transfer request. In this manner, data transfer between the receiver stations is carried out in order to recover missing delivery data. This configuration avoids a situation where a large number of transfer requests made by receiver stations concentrate on the single transmission station.
Further, the delivery data are transferred between the receiver stations via a satellite communication link or a surface communication line. This insures successful data recovery even if a surface communication line of the transmission station suffers a connection failure.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.